1. Field of the Invention
The present invention relates to an apparatus for detecting a focussing position which is well suited for use in a focussing system for automatically compensating the focussing position of an image of a mask pattern on the surface of a wafer in a projection exposure apparatus used in the manufacturing process of semiconductor devices or the so-called automatic focussing system.
2. Description of the Prior Art
Projection exposure techniques of arranging a mask pattern and a stage through the intermediary of a projection optical system and transferring the mask pattern onto a photosensitive substrate (wafer) on the stage have been put in practical use in various fields of precision processings such as the fabrication of semiconductor devices, and a variety of projection exposure apparatus adapted for various applications in these fields have been available on the market. In the case of these projection exposure apparatus, it is an essential requirement that the exposure surface of a photosensitive substrate must be coincident with a conjugate position (height) of a mask pattern relative to the projection optical system, that is, the focal point of the mask pattern must be coincident with the exposure surface under the application of the actual exposure light. As a result, it has been the usual practice that the projection exposure apparatus for ordinary production purposes incorporates a focussing system which automatically effects the required focussing or the so-called automatic focussing system so as to realize the consistency of both the efficiency and reliability of the projection exposure.
While, as one method for such focussing system, a system has also been reported in which as disclosed for example in Japanese Laid-Open Patent Publication No. 57-212406, a special mark formed on the surface of a mask pattern is directly projected onto the exposure surface of a photosensitive substrate and the resulting projected image is optically detected through the projection optical system and the mark thereby directly discriminating the focussing position, generally a system has been employed in which means A for directly detecting the focussing of the mask pattern surface on a reference surface formed on the stage by use of the exposure light is combined with means B which is adapted to measure the stage-side height below the projection optical system and zero-adjusted to the focussing height by means of the means A whereby the height of the exposure surface is indirectly detected by use of the measuring means B thereby guiding the exposure surface up to the focussing height. Then, as the means A utilizing the exposure light, as for example, Japanese Laid-Open Patent Publication No. 1-286418 discloses a system in which a special mark formed on the surface of a mask pattern is projected onto a reference surface. According to this system, the projected image of the mark formed on the reference surface is observed through the projection optical system and the mark so that the light quantity peak of the projected image regulated by the mark is detected thereby discriminating the focussing. Also, as an example of the measuring means B for measuring the stage-side height, as for example, Japanese Laid-Open Patent Publication No. 1-04962 discloses a system in which the height of the exposure surface just below the projection optical system is measured by using an optical system obliquely attached to the outer side of the projection optical system.
Presently, in the case of semiconductor memory devices which are particularly high in processing accuracy, the projection with a depth of focus of about 1 .mu.m is effected by using the i-line of 365 nm in wavelength and the focussing positioning accuracy of 0.1 .mu.m or less is usually required, whereas an extremely high accuracy of 0.05 .mu.m or less is required in the case of a special projection exposure utilizing an interference phenomenon of the exposure light such as the one disclosed in Japanese Laid-Open Patent Publication No. 62-50811. In order to satisfy such high degree of accuracy, it is no longer possible to ignore any small bending and slanting of a mask itself with the result that such high accuracy cannot be dealt with by the conventional method of detecting the focussing position of a mark provided in one corner of a mask pattern and causing it to serve as the focussing position of the mask pattern on the whole and thus it is required that each of the portions in the photosensitive substrate-side exposure area is adjusted up to the focussing height in one-to-one correspondence to one of the portions within the transfer area of the mask pattern. Thus, a method has been conceived in which the focussing position is obtained for each of the portions within the transfer area of the mask pattern and the variations in the focussing position (height) of the respective partial areas are absorbed as far as possible by slanting the stage.
While, in this method, the focussing height is obtained as a matter of course for each of the portions in the actual mask pattern immediately before the exposure and transfer, this cannot be performed by the conventional method of directly detecting the focussing position through the use of the exposure light. For example, in order to perform the system disclosed in the previously mentioned Japanese Laid-Open Patent Publication No. 1-286418, it is necessary to arrange a focussing position detection mark on each of the portions within the mask pattern transfer area and this is an unfeasible since it produces a detrimental effect on the level of integration of mask patterns.
Also, in accordance with the systems of the previously mentioned Japanese Laid-Open Patent Publications No. 57-212406 and No. 1-286418 the optical information from a projected image on an exposure surface goes back partially through the optical path of an exposure light and reaches a detector, with the result that the reflected light of the exposure light in the optical path section, e.g., the exposure light reflected from the surfaces of the lenses in the projection optical system, the back side of the mask pattern and the both sides of the mask is returned to reach as such up to the detector and therefore the detector must subtracts a huge beckground due to the reflected light of the exposure light to detect the peak of the optical information. As a result, in the case of (a) where the area of the mark accounts for a small part of the illumination field (the mask pattern) of the exposure light, (b) where the light shielding area accounts for a large part of the mask pattern (the amount of reflection from the back side of the mask pattern is large) or (c) where the output of the light source varies causing the intensity of the exposure light to vary moment by moment, the peak detection by the detector becomes difficult and there is the possibility of the discrimination of the focussing position failing to function properly.